As the use of electronic devices that transmit and receive radio frequency signals increases, so does the problem of isolating signals of interest in interference. This is particularly true in dense urban environments where frequency reuse is becoming common and more tightly packed users of radio spectrum compete for finite bands of spectrum. Cities, for example, may be home to many transmitters using the Wi-fi, Wi-max, TV “white space” bands, etc. Given the close proximity of these transmitters to one another, isolating a particular transmitter of interest among interfering signals is a challenge.
An illustrative example of the problem is cellular towers. Cellular towers in a city may re-use the same frequency on towers arranged in a grid only a mile apart. In a city of one hundred square miles, there may be as many as 100 cellular towers operating on the same frequency. In this simple example, the desired signal, that is a hypothetical signal of interest, is almost never the strongest signal on a particular frequency. There could be 99 other interfering signals to contend with. Additionally, the signal of interest may be so weak as to be below the receiver's noise level if a low-gain antenna is used.
One conventional solution to the problem of how to “dig” a signal of interest out of interfering signals is the use of adaptive beam-forming and interference cancelling antennas. Such antennas are conventionally constructed of multiple spaced-apart antenna elements. The relative location of all the elements of a conventional array are tightly fixed and well-characterized. The time and/or phase delay between conventional antenna array elements is also well-characterized.
For a conventional antenna array, a signal from a given transmitter is received at the various antenna elements. The signal as it is received at the various antenna elements is time-delayed (or equivalently, for narrow-band signals, experiences a phase shift) according to the amount of distance the signal had to travel from the transmitter to the various antenna elements. When the signals from the various antenna elements in the conventional array are summed, the signals from the various antenna elements can interfere either destructive or constructively. The delay between antenna elements can be controlled, either by the physical spacing between the elements, or by the addition of delay elements, to provide constructive additive combination to occur for signals from one location, while destructive additive combination (nulls) occur for signals from other locations.
In this way, conventional antenna arrays have been constructed where a beam (that is, a direction for which signals will be constructively added) can be formed and pointed in a desired direction. This improves the signal-to-white noise power ratio by the number of antenna elements coherently combined. However, interfering signals can still enter through the array sidelobes and the edge of the main beam. One way to cancel interfering signals in conventional antenna arrays is to form multiple beams orthogonal to the beam pointed at the target. The beam and its orthogonal beams are then adaptively combined with a feedback circuit controlling the gain and phase weighting of the many beams to form nulls in the composite pattern of energy that are not co-located with a location along the desired direction.
The disadvantage of conventional antenna arrays is the calibration required of the array. For conventional antenna arrays, the gain and phase characteristics of the antenna elements and the receiver channels must be known. In order to form beams with −20 dB nulls at specific spatial locations, calibration to approximately 6 degrees in phase and 10% in amplitude is generally required. Deeper nulls require even more precise phase and amplitude calibration. This is achieved conventionally by careful attention to receiver phase properties and inserting calibration signals immediately after the antenna elements to calibrate the respective receiver channels. Likewise, the placement of the antenna elements and multipath reflections must be carefully controlled.
The elaborate calibration and tightly controlled placement necessary for the operation of conventional antenna arrays is complex and expensive. What is needed is a method of picking an individual signal out of a crowded frequency space with an array of arbitrary receivers whose relative position and phase characteristics are not known a priori.